Population Ecology: Agriculture Nature and Importance

Prepared by: Syed Ijlal Ahmed Waleed


In previous chapters/units we have concentrated on the biology of the individual cell, tissue, and organism. There are levels of organization above the individual organism that will be the subject of this unit. Individual organisms are grouped into populations, which in turn form communities, which form ecosystems. Ecosystems make up the biosphere, which includes all life on Earth. If there is life on other planets, will we need another level of organization?

Biosphere: The sum of all living things taken in conjunction with their environment. We divide the earth into atmosphere (air), lithosphere (earth), hydrosphere (water), and biosphere (life).

Ecosystem: The relationships of smaller groups of organisms with each other and their environment. Scientists often speak of the interrelatedness of living things. Since, according to Darwin’s theory, organisms adapt to their environment, they must also adapt to other organisms in that environment. We can discuss the flow of energy through an ecosystem from photosynthetic autotrophs to herbivores to carnivores.

Community: The relationships between groups of different species. For example, the desert communities consist of rabbits, coyotes, snakes, birds, mice and such plants as sahuaro cactus (Carnegia gigantea), Ocotillo, creosote bush, etc. Community structure can be disturbed by such things as fire, human activity, and over-population.

Species: Groups of similar individuals who tend to mate and produce viable, fertile offspring. We often find species described not by their reproduction (a biological species) but rather by their form (anatomical or form species).

Populations: Groups of similar individuals who tend to mate with each other in a limited geographic area. This can be as simple as a field of flowers, which is separated from another field by a hill or other area where none of these flowers occur.

Individuals: One or more cells characterized by a unique arrangement of DNA “information”. These can be unicellular or multicellular. The multicellular individual exhibits specialization of cell types and division of labor into tissues, organs, and organ systems.

Organ System: (in multicellular organisms). A group of cells, tissues, and organs that perform a specific major function. For example: the cardiovascular system functions in circulation of blood.

Organ: (in multicellular organisms). A group of cells or tissues performing an overall function. For example: the heart is an organ that pumps blood within the cardiovascular system.

Tissue: (in multicellular organisms). A group of cells performing a specific function. For example heart muscle tissue is found in the heart and its unique contraction properties aid the heart’s functioning as a pump. .

Cell: The fundamental unit of living things. Each cell has some sort of hereditary material (either DNA or more rarely RNA), energy acquiring chemicals, structures, etc. Living things, by definition, must have the metabolic chemicals plus a nucleic acid hereditary information molecule.

Organelle: A subunit of a cell, an organelle is involved in a specific subcellular function, for example the ribosome (the site of protein synthesis) or mitochondrion (the site of ATP generation in eukaryotes).

Molecules, atoms, and subatomic particles: The fundamental functional levels of biochemistry.
Organization levels of life, in a graphic format.  It is thus possible to study biology at many levels, from collections of organisms (communities), to the inner workings of a cell (organelle).

Ecology is the study how organisms interact with each other and their physical environment. These interactions are often quite complex. Human activity frequently disturbs living systems and affects these interactions. Ecological predictions are, of a consequence, often more general than we would like.

A population is a group of individuals of the same species living in the same geographic area. The study of factors that affect growth, stability, and decline of populations is population dynamics. All populations undergo three distinct phases of their life cycle:
1.    growth
2.    stability
3.    decline

Population growth occurs when available resources exceed the number of individuals able to exploit them. Reproduction is rapid, and death rates are low, producing a net increase in the population size.

Population stability is often proceeded by a “crash” since the growing population eventually outstrips its available resources. Stability is usually the longest phase of a population’s life cycle.

Decline is the decrease in the number of individuals in a population, and eventually leads to population extinction.

Nearly all populations will tend to grow exponentially as long as there are resources available. Most populations have the potential to expand at an exponential rate, since reproduction is generally a multiplicative process. Two of the most basic factors that affect the rate of population growth are the birth rate, and the death rate. The intrinsic rate of increase is the birth rate minus the death rate.
Two modes of population growth. The Exponential curve (also known as a J-curve) occurs when there is no limit to population size. The Logistic curve (also known as an S-curve) shows the effect of a limiting factor (in this case the carrying capacity of the environment).

The age within its individual life cycle at which an organism reproduces affects the rate of population increase. Life history refers to the age of sexual maturity, age of death, and other events in that individual’s lifetime that influence reproductive traits. Some organisms grow fast, reproduce quickly, and have abundant offspring each reproductive cycle. Other organisms grow slowly, reproduce at a late age, and have few offspring per cycle. Most organisms are intermediate to these two extremes.

Population curves. a) three hypothetical populations (labeled I, II, and III); b, c, and d) three real populations. Note that the real curves approximate one of the three hypotheticals. Age structure refers to the relative proportion of individuals in each age group of a population. Populations with more individuals aged at or before reproductive age have a pyramid-shaped age structure graph, and can expand rapidly as the young mature and breed. Stable populations have relatively the same numbers in each of the age classes.
Comparison of the population age structuire in the United States and Mexico. Note the deographic bulge in the Mexican population. The effects of this buldge will be felt for generations.

Human populations are in a growth phase. Since evolving about 200,000 years ago, our species has proliferated and spread over the Earth. Beginning in 1650, the slow population increases of our species exponentially increased. New technologies for hunting and farming have enabled this expansion. It took 1800 years to reach a total population of 1 billion, but only 130 years to reach 2 billion, and a mere 45 years to reach 4 billion.
Despite technological advances, factors influencing population growth will eventually limit expansion of human population. These will involve limitation of physical and biological resources as world population increased to over six billion in 1999. The 1987 population was estimated at a puny 5 billion.


Human population growth over the past 10,000 years. Note the effects of worldwide disease (the Black death) and technological advances on the populatiuon size.

Limits on population growth can include food supply, space, and complex interactions with other physical and biological factors (including other species). After an initial period of exponential growth, a population will encounter a limiting factor that will cause the exponential growth to stop. The population enters a slower growth phase and may eventually stabilize at a fairly constant population size within some range of fluctuation.This model fits the logistic growth model. The carrying capacity is the point where population size levels off.

Relationship between carrying capacity (K) and the population density over time.

The environment is the ultimate cause of population stabilization. Two categories of factors are commonly used: physical environment and biological environment. Three subdivisions of the biological environment arecompetition, predation, and symbiosis.

Physical environment factors include food, shelter, water supply, space availability, and (for plants) soil and light. One of these factors may severely limit population size, even if the others are not as constrained. TheLaw of the Minimum states that population growth is limited by the resource in the shortest supply.

The biological role played by a species in the environment is called a niche. Organisms/populations in competition have a niche overlap of a scarce resource for which they compete. Competitive exclusion occurs between two species when competition is so intense that one species completely eliminates the second species from an area. In nature this is rather rare. While owls and foxes may compete for a common food source, there are alternate sources of food available. Niche overlap is said to be minimal.

Paramecium aurelia has a population nearly twice as large when it does not have to share its food source with a competing species. Competitive release occurs when the competing species is no longer present and its constraint on the winner’s population size is removed.

Graphs showing competition between two species of Paramecium. Since each population alone prospers (yop two graphs), when they are in a competition situation one species will win, the other will lose (bottom graph).

Predators kill and consume other organisms. Carnivores prey on animals, herbivores consume plants. Predators usually limit the prey population, although in extreme cases they can drive the prey to extinction. There are three major reasons why predators rarely kill and eat all the prey:
1.    Prey species often evolve protective mechanisms such as camouflage, poisons, spines, or large size to deter predation.
2.    Prey species often have refuges where the predators cannot reach them.
3.    Often the predator will switch its prey as the prey species becomes lower in abundance: prey switching.
Fluctuations in predator (wolf) and prey (moose) populations over a 40-year span. Note the effects of declines in the wolf population in the late 1960s and again in the early 1980s on the moose population.

Symbiosis has come to include all species interactions besides predation and competition. Mutualism is a symbiosis where both parties benefit, for example algae (zooxanthellae) inside reef-building coral. Parasitism is a symbiosis where one species benefits while harming the other. Parasites act more slowly than predators and often do not kill their host. Commensalism is a symbiosis where one species benefits and the other is neither harmed nor gains a benefit: Spanish moss on trees, barnacles on crab shells. Amensalism is a symbiosis where members of one population inhibit the growth of another while being unaffected themselves.

Natural populations are not governed by a single control, but rather have the combined effects of many controls simultaneously playing roles in determining population size. If two beetle species interact in the laboratory, one result occurs; if a third species is introduced, a different outcome develops. The latter situation is more like nature, and changes in one population may have a domino effect on others.

Which factors, if either is more important in controlling population growth: physical or biological? Physical factors may play a dominant role, and are called density independent regulation, since population density is not a factor. The other extreme has biological factors dominant, and is referred to as density dependent regulation, since population density is a factor. It seems likely that one or the other extreme may dominate in some environments, with most environments having a combination control.

Extinction is the elimination of all individuals in a group. Local extinction is the loss of all individuals in a population. Species extinction occurs when all members of a species and its component populations go extinct.Scientists estimate that 99% of all species that ever existed are now extinct. The ultimate cause of decline and extinction is environmental change. Changes in one of the physical factors of the environment may cause the decline and extinction; likewise the fossil record indicates that some extinctions are caused by migration of a competitor.

Dramatic declines in human population happen periodically in response to an infectious disease. Bubonic plague infections killed half of Europe’s population between 1346 and 1350, later plagues until 1700 killed one quarter of the European populace. Smallpox and other diseases decimated indigenous populations in North and South America.

Human populations have continued to increase, due to use of technology that has disrupted natural populations. Destabilization of populations leads to possible outcomes:
•    population growth as previous limits are removed
•    population decline as new limits are imposed

Agriculture and animal domestication are examples of population increase of favored organisms. In England alone more than 300,000 cats are put to sleep per year, yet before their domestication, the wild cat ancestors were rare and probably occupied only a small area in the Middle East.

Pollutants generally are (unplanned?) releases of substances into the air and water. Many lakes often have nitrogen and phosphorous as limiting nutrients for aquatic and terrestrial plants. Runoff from agricultural fertilizers increases these nutrients, leading to runaway plant growth, or eutrophication. Increased plant populations eventually lead to increased bacterial populations that reduce oxygen levels in the water, causing fish and other organisms to suffocate.

Removal of a competing species can cause the ecological release of a population explosion in that species competitor. Pesticides sprayed on wheat fields often result in a secondary pest outbreak as more-tolerant-to-pesticide species expand once less tolerant competitors are removed.

Predator release is common where humans hunt, trap, or otherwise reduce predator populations, allowing the prey population to increase. Elimination of wolves and panthers has led to increase in their natural prey: deer. There are more deer estimated in the United States than there was when Europeans arrived. Large deer populations often cause over grazing that in turn leads to starvation of the deer.

Introduction of exotic or alien non-native species into new areas is perhaps the greatest single factor to affect natural populations. More than 1500 exotic insect species and more than 25 families of alien fish have been introduced into North America; in excess of 3000 plant species have also been introduced. The majority of accidental introductions may fail, however, once an introduced species becomes established, its population growth is explosive. Kudzu, a plant introduced to the American south from Japan, has taken over large areas of the countryside.
Kudzu covering a building (left) and closeup of the flowers and leaves (right).

Humans can remove or alter the constraints on population sizes, with both good and bad consequences. On the negative side, about 17% of the 1500 introduced insect species require the use of pesticides to control them. For example, African killer bees are expanding their population and migrating from northward from South America. These killer bees are much more agressive than the natives, and destroy native honeybee populations.

On a positive note, human-induced population explosions can provide needed resources for growing human populations. Agriculture now produces more food per acre, allowing and sustaining increased human population size.

Human action is causing the extinction of species at thousands of times the natural rate. Extinction is caused by alteration of a population’s environment in a harmful way. Habitat disruption is the disturbance of the physical environment of a species, for example cutting a forest or draining wetlands. Habitat disruption in currently the leading cause of extinction.

Changes in the biological environment occur in three ways.
1.    Species introduction: An exotic species is introduced into an area where it may have no predfators to control its population size, or where it can gratly out compete native organisms. Examples include zebra mussels introduced into Lake Erie, and lake trout released into Yellowstone Lake where they are threatening the native cutthroat trout populations.
2.    Overhunting: When a predator population increases or becomes more efficient at killing the prey, the prey population may decline or go extinct. Examples today include big game hunting, which has in many places reduced the predator (or in this case prey) population. In human prehistory we may have caused the extinction of the mammoths and mastodons due to increased human hunting skill.
3.    Secondary extinction: Loss of food species can cause migration or extinction of any species that depends largely or solely on that species as a food source.

Overkill is the shooting, trapping, or hunting of a species usually for sport or economic reasons. Unfortunately, this cannot eliminate “pest” species like cockroaches and mice due to their large population sizes and capacity to reproduce more rapidly than we can eliminate them. However, many large animals have been eliminated or had their populations drastically reduced (such as tigers, elephants, and leopards).

The death of one species or population can cause the decline or elimination of others, a process known as secondary extinction. Destruction of bamboo forests in China, the food for the giant panda, may cause the extinction of the panda. The extinction of the dodo bird has caused the Calviera tree to become unable to reproduce since the dodo ate the fruit and processed the seeds of that tree.

Giant pandas eat an estimated 10,000 pounds of bamboo per panda per year.

Even if a number of individuals survive, the population size may become too small for the species to continue. Small populations may have breeding problems. They are susceptible to random environmental fluctuations and genetic drift to a greater degree than are larger populations. The chance of extinction increases exponentially with decreasing population size.

The minimum viable population (MVP) is the smallest population size that can avoid extinction by the two reasons listed above. If no severe environmental fluxes develop for a long enough time, a small population will recover. The MVP depends heavily on reproductive rates of the species.

Populations tend to have a maximum density near the center of their geographic range. Geographic range is the total area occupied by the species. Outlying zones, where conditions are less optimal, include the zone of physiological stress (where individuals are rare), and eventually the zone of intolerance (where individuals are not found).

Integrated pest management emphasizes the importance of interactions between pests and the natural enemies that prey upon them. When broad-spectrum insecticides are applied, pest and non-pest species are killed and the balance of the community is disrupted. For example, pesticide use in pear orchards to control codling moth can also destroy the natural enemies of pear psylla. In the absence of its natural enemies, pear psylla can reach high densities and cause significant damage to the fruit.

Natural enemies are divided into two main groups: predators and parasites. A predator lives by capturing and feeding on another species. Predators are usually larger and more powerful than their prey. Many of the most common predators in fruit production systems attack a wide range of pest species and help regulate pest population densities.

Common predators and some of their prey in fruit crops
Predators Prey
Amoebae Soilborne fungi, bacteria
Anthocorid bugs Spider mites, thrips, aphids, pear psylla, young scale, various insect eggs
Bigeyed bugs Lygus bugs, aphids, leafhoppers, spider mites
Collembola Fungi
Ladybird beetles Aphids, scale insects, pear psylla, mealybugs, other soft-bodied prey
Lacewings Aphids, scale insects, mealybugs, pear psylla leafhoppers, thrips, mites
Mirid bugs Spider mites, aphids, leafhoppers, pear psylla, scale insects
Mycophagous mites Fungi, eg. grapevine powdery mildew
Nematodes Soilborne fungi, bacteria, other nematodes
Predatory mites Plant-feeding mites
Spiders Pear psylla, aphids, leafhoppers
Syrphid flies or flower flies Aphids, scale insects
Common parasites and some of their hosts in fruit crops
Parasites Hosts
Aphelinid wasps Aphids
Tachinid flies Caterpillars, beetles
Trichogramma wasps Moth eggs
Bacillus thuringiensis (bacterium) Butterfly/moth larvae
Pseudomonas fluorescens (bacterium) Fungi
Polyhedrosis virus Butterfly/moth larvae
Beauveria bassiana (fungus) Many insects
Trichoderma harzianum (fungus) Pythium, Rhizoctonia and other pathogens
Ampelomyces quisqualis (fungus) Powdery mildew
Arthrobytris (nematode-trapping fungus) Nematodes
Steinernema (nematode) Insect larvae
Pasteuria penetrans (bacterium) Nematodes

A parasite lives in, on, or with another organism and obtains food and usually shelter at the host’s expense. Parasitic insects and microbes are important in the biological control of many pests. Plant pathogens may be considered parasites that cause disease symptoms in plants.

An insect that is parasitic on other insects during its immature stages, but is free-living as an adult, is called a parasitoid. Most parasitoids are small flies or wasps. Parasitoids are often common in flowering plants such as fruit crops and therefore are potentially very beneficial allies of fruit growers. Some parasitoids are specialists, attacking one or a few host species, while a few are generalists and use a wide variety of other insects as hosts. The free-living adults often feed on the nectar provided by flowers. The female parasitoid finds a host and lays eggs. The parasitoid larva develops inside or on the host. At first the larva feeds only on fatty tissues, allowing the host to continue to grow and develop. As the parasitoid nears the end of its development, it consumes the host’s vital organs, killing it. The parasitoid larva pupates and later emerges as an adult.

The emerging parasitoid often leaves behind telltale signs of its handiwork. When scouting for pests, also watch for parasitoid pupal cases or emergence holes in insect bodies. Try to choose management strategies that protect parasitoids such as using selective insecticides.

The environment is usually never uniform enough to support uniform distribution of a species. Species thus have a dispersion pattern. Three patterns found include uniform, clumped, and random.

Geographic ranges of species are dynamic; over time they can contract or expand due to environmental change or human activity. Often a species will require another species’ presence, for example Drosophila in Hawaii. Species ranges can also expand due to human actions: brown trout are now found worldwide because of the sprerad of trout fishing.

Some predators and parasitoids use alternative food sources during the growing season. These include prey or hosts other than pests, and nectar producing plants other than fruit crops. If an alternative host is not available, the predator or parasitoid may not survive or stay long enough in the crop to control pests when needed. Predatory mites often feed on rust mites when their primary hosts, spider mites, are absent or in low numbers. Parasitoids may require an alternative host to complete their life cycle. A small parasitic wasp, Colpoclypeus florus, can have a major impact on some leafroller populations in apple orchards. However, C. florus is often not present in high numbers early in the season because none of the leafrollers in the orchard overwinter as late instar larvae, the host size required to complete its development. Another leafroller host of this parasitic wasp overwinters as a large larva on wild rose, found in wild habitats around orchards. Larvae of the parasitoid successfully overwinter in this host on rose and complete their development early in the spring. They then emerge and can fly to colonize leafrollers in nearby orchards.

Many natural enemies require more than one kind of food to develop normally and sustain their populations. Syrphid fly adults supplement their diets by gathering and eating pollen from flowering plants. Where natural enemies have access to pollen and nectar, there is often more predation and lower abundance of pests. Similarly, parasitic wasps supplement their diets by feeding on nectar, aphid honeydew, and other sources of sugar. For example, Trichogramma species are tiny wasps that parasitize the eggs of moths, such as codling moth. Planting a cover crop that includes flowering plants is a good way to provide nectar sources for these beneficial insects, but care must be taken in selecting a cover crop that is not a host for other pests.

Some pathogens also need to find alternative hosts when a fruit host is unavailable. For example, the root knot nematode can also reproduce on dandelions in vineyards. This weed can also serve as a host for viruses that are vectored by the dagger nematode. Weed management is essential to reduce these types of risk. Cover crops that suppress weeds and nematodes have been used in fruit systems and are likely to play an even more important role in the future. Verticillium albo-atrum, a soilborne fungus that causes a severe wilt in strawberries, also attacks the roots of many other hosts, especially solanaceous crops like tomatoes, peppers, and potatoes. Growers are advised not to plant strawberries after Verticillium-susceptible crops.

Management practices change the dynamics of the community of pests and natural enemies within the crop. The positive effects like reduced pest numbers and increased yields are obvious. Certain management decisions, however, can have unintended impacts on the community.

Impact of cultural practices:
Fire blight of apple and pear was once considered a sporadic disease that usually could be managed by combining cultural and chemical methods. Since the early 1990’s, however, fire blight has plagued apple growers to a degree previously unknown. Indeed, modern fire blight epidemics have been an economic disaster. The “new face” of fire blight has resulted from several changes in the apple crop habitat. Genetic, physical, and cultural factors have interacted to create ideal conditions for growth and spread of the fire blight pathogen:
•    The number of trees planted per acre has increased dramatically. This means that fire blight can move more easily from tree to tree.
•    More acres are being planted to highly susceptible cultivars, including Braeburn, Fuji, Jonathan, and Rome.
•    Size-controlling rootstocks, many of which are highly susceptible to fire blight, are used to achieve high-density plantings.
•    Trees are being pushed to bear earlier and training systems are being adopted that are very different from the way apple trees grow in nature. These modern orchards have a high density of apple tissues such as flowers and young shoots that are very susceptible to fire blight. Traditional, low-density orchards also have susceptible tissues, but they are interspersed with a lot of older, woody tissues that are much less favorable to the growth of the fire blight pathogen. New tools are being developed to manage fire blight on susceptible varieties in high-density plantings. For example, there are a few size-controlling rootstocks that are relatively resistant to fire blight. Certain plant growth regulators reduce vigorous shoot growth and thereby reduce shoot susceptibility to fire blight. Also, larger trees planted at lower densities will not have the production potential of the more modern orchard systems, but they will be more likely to survive fire blight long enough to yield a crop.

Although pesticide sprays are generally targeted against one or a few pest populations, they often influence other pest and non-pest species. Some insecticides are very toxic to predators and parasitoids. Destroying these natural enemies often results in target pest resurgence or secondary pest outbreaks. Some pesticides have a greater impact on the natural enemies than the target pest. Target pest resurgence can result when the unfavorable ratio of pests to natural enemies permits a rapid increase or resurgence of the pest population. For example, biological control of twospotted spider mite by predatory mites is common in many fruit crops. Insecticides that are applied for control of pest mites and insects are often highly toxic to predatory mites. Some pest mites survive the spray, but most predators are killed. The population of twospotted spider mites is able to quickly rebound, reaching economically damaging levels before its natural enemy can re-colonize from unsprayed areas.

Pre-plant fumigation of the soil is often used for control of black root rot in strawberry. However, if black root rot pathogens are re-introduced in fumigated soil with infected planting material, the disease comes back with a vengeance, presumably because competitive organisms in the soil have been eliminated by the fumigation.

A secondary pest outbreak occurs when a pesticide that was applied to control one pest kills the natural enemies that were keeping a second pest population in check. For example, a complex of predators can be helpful in keeping aphid populations from reaching damaging levels. The broad-spectrum insecticides that are used to control key pests are highly toxic to these predators. As a result, applying one of these insecticides often leads to secondary outbreaks of aphid populations. Pesticide applications can also impact beneficial microbes leading to increased plant disease problems.

Secondary pest problems are not always associated with the destruction of natural enemies. Control of codling moth by mating disruption entails releasing enough sex pheromone into orchards to interfere with mate location, reducing reproduction and subsequent larval infestations. However, using this highly specific tactic in place of broad-spectrum insecticides can also have a significant impact on other potential pests. In disrupted orchards, leafrollers that were kept at non-damaging levels by broad-spectrum insecticides are now only suppressed by natural enemies. Unless natural controls provide sufficient suppression, leafroller populations will increase, sometimes reaching damaging levels.

In similar fashion, minor diseases that are not normally a problem can become important when major diseases are controlled. Since pathogens often compete for space and nutrients, removing one pathogen with a fungicide may benefit other pathogens that are not affected by that particular fungicide. An example of this is the increase in Alternaria infections of blueberry fruit in fields that have been treated with a fungicide against anthracnose fruit rot.

Reference http://www.grapes.msu.edu/pestInteract.htm

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